The present invention relates in general to electronic circuits and, more particularly, to depletion mode metal oxide semiconductor (MOS) devices in a series configuration.
Supervisor circuits find use in most if not all system supervisor applications. Systems requiring supervisor circuits are diverse and cover a wide range of uses from battery-powered equipment, computers, embedded systems, and microprocessor power supply monitoring. The supervisor circuit monitors a supply voltage and provides a reset signal to the microprocessor in the presence of a low supply voltage. If the supply voltage falls below some specified operating threshold the supervisor circuit triggers a reset of the microprocessor.
Typically, the supervisor circuit consist of a comparator to monitor changes in the supply voltage, and a trigger circuit to reset the microprocessor upon a supply voltage drop. If the comparator detects a change in supply voltage with respect to a voltage reference, the comparator sends a signal to the trigger circuit to reset the microprocessor.
As an example, a supervisor circuit monitors the supply voltage line of a microprocessor and compares the supply voltage to a voltage reference. If the supply voltage to the microprocessor drops below the voltage reference the supervisor circuit detects the voltage drop and resets the microprocessor, and puts the microprocessor in a fault mode. The microprocessor stays in the fault mode until the supply voltage increases above the voltage reference, at which time the supervisor circuit removes the reset and the microprocessor thereafter operates in a normal mode.
The output of current prior art supervisor circuits have a state at which it can not guarantee an output if the supply voltage drops below some threshold voltage. The point below the threshold voltage at which the output of the supervisor circuit is not guaranteed is dependent upon the minimum voltage required to operate the trigger circuit. Typical implementations of a trigger circuit in the prior art are open drain or complimentary transistor configurations. Both of these configurations require a minimum voltage to be applied to the devices before the devices are operative. As a result, if the supply voltage to the microprocessor drops below the minimum voltage required to operate the trigger circuit, the output of the supervisor circuit can not be guaranteed and the microprocessor may inadvertently fall out of a fault mode. Also, for low voltage applications the supply voltage may operate close to the minimum voltage required to operate the trigger circuit, again resulting in an operation where the microprocessor may fall out of a fault mode.
Low voltage applications typically require linear regulators. A linear regulator takes in an unregulated input voltage and converts it to a regulated output voltage. The prior art typically uses an enhancement mode p-type transistor and a controller to provide the regulation. When the enhancement mode p-type transistor is turned off by the controller, there is some leakage current through the enhancement mode p-type transistor. The leakage current is typically inversely related to the threshold voltage of the enhancement mode p-type transistor. Thus, you have to make the threshold voltage of the enhancement mode p-type transistor higher to reduce the leakage current. However, the input voltage must be at least the threshold voltage of the enhancement mode p-type transistor to turn the linear regulator on. Therefore, to operate a linear regulator at a low voltage results in a high leakage current through the device.
With advancing technology, most supply voltages to systems will drop down to the minimum voltage required to operate current prior art enhancement mode devices. For example, if the supply voltage of a microprocessor operates close to the minimum voltage required to operate the trigger circuit the microprocessor may experience inadvertent fault or operating modes. Furthermore, it would not be possible for a system to have a linear regulator operate at a low voltage with little or no leakage current using prior art enhancement mode devices. Thus, to meet advancing technology, a need exists for a circuit which can operate close to zero volts to replace trigger circuits presently used in prior art supervisor circuits to guarantee an output of the supervisor circuit for all input voltage levels. Also, the circuit must provide a linear regulator to operate in a low voltage environment without a high leakage current as seen with current prior art technology.